Increasing the efficiency of internal combustion engines, particularly automobile and truck engines, has been a continuing challenge to engineers and designers alike ever since the advent of the engine itself. In this area, much progress has been made in increasing the efficiency with which fuel is burned in such engines as by improvements in carburetors, air-fuel mixing systems, and fuel vaporizers. However, little progress has been made in successfully recovering and utilizing the waste heat from such internal combustion engines, particularly the waste heat in their exhaust gases and coolant which can typically represent 60% or more of the energy available from the burned fuel. As compared to systems that concentrate on increasing the efficiency of the fuel burning process within the engine, waste heat recovery systems must be more precisely designed if any significant gains are to be achieved because much of the energy available in the waste heat is at a lower energy level. Further, to achieve any worthwhile gains, a waste heat recovery system must not only recover as much of the waste heat as possible but also return it to the power output of the engine in a direct and efficient manner.
By far, the vast majority of waste heat recovery systems for internal combustion engines use a single circuit of working fluid (typically freon) wherein waste heat from the engine's lubricant, coolant, and/or exhaust is progressively added in steps to the freon. Examples of such systems are U.S. Pat. Nos. 3,228,189 to Baker; 3,554,849 to Wagner; 3,888,084 to Hawkins; 3,945,210 to Chapin; 4,003,034 to Bradley; 4,069,672 to Milling; and 4,099,489 to Bradley. Such systems are fundamentally defective in design because the recovered heat from the lubricant and coolant is not productively used. Specifically, the difference in available energy between the exhaust gases and the lubricant and coolant is so great that virtually all of the energy added to the freon is from the exhaust and the heat exchanges with the lubricant and the coolant might as well be eliminated. Also, since the exhaust is fully capable of heating the freon to its maximum operating temperature by itself, this pre-heating by the lubricant and coolant means that less heat is extracted from the hot exhaust and more of the energy of the hot exhaust is inefficiently expelled into the atmosphere. Further, most such systems use the recovered heat to drive turbines which must be operated at such high speeds to be efficient that a gear down arrangement must be provided to couple the work of the turbine to the drive or crank shaft of the typical engine. As a practical matter, this is somewhat of a self-defeating proposition since much of the gain from the turbine is lost in the added weight and operation of the gear down arrangement.
Several patented systems such as U.S. Pat. No. 4,031,705 to Berg and U.S. Pat. No. 3,830,062 to Morgan are of interest for their realization of the fundamental defects in the series type recovery systems; however, none of these is really an efficient, workable alternative. Berg, for example, has parallel heat exchangers for the exhaust and coolant but then recombines the heated freon before extracting work from it. The real effect of this recombining is to reduce the temperature of the freon from the exhaust exchanger and thus reduce the temperature and energy of the freon entering the expander. Like Berg, Morgan has parallel heat exchangers for the exhaust and coolant but unlike Berg, Morgan also has parallel turbines and does not recombine the exhaust and coolant heated freon before extracting work from it. Still, Morgan's design is deficient in several areas especially if it were to be applied to automobile and truck engines. Notably, he wastes energy pre-heating the freon prior to its entering the exhaust heat exchangers. This pre-heating prior to the freon entering the exhaust heat exchanger is unnecessary since the waste heat from the exhaust can more than adequately heat the freon to its maximum operating temperature by itself. Further, this pre-heating means that less heat is extracted from the hot exhaust and more of the energy of the hot exhaust is inefficiently expelled and lost to the atmosphere. Other patents in this general field are U.S. Pat. Nos. 2,919,540 to Percival; 3,948,235 to Gamell; 3,979,913 to Yates; 4,007,594 to Elsea; and 4,120,157 to Tang.
It was with the drawbacks and deficiencies of systems such as these in mind that the present waste heat recovery system was developed. In the present system, two separate and closed circuits of working fluids are used for maximum efficiency. One circuit operates off a heat exchanger with the exhaust and the other operates off a heat exchanger with the coolant. For increased efficiency, each circuit has a different working fluid and operates at temperatures and pressures most efficient for that particular fluid and the heat available from the exhaust in one case and the coolant in the other case. There is also a heat exchanger between the two circuits for further efficiency and a direct coupling of the work produced from each circuit to the power output of the engine. In the preferred embodiment, the work produced by the exhaust heated circuit is sequentially added to the power output of the engine and a heat reservoir of melted salt is built into the exhaust heated circuit to minimize surges in the system and to provide power during high performance demands. A manner is also disclosed in which the basic flow pattern of the preferred embodiment can be adapted to operate with a single working fluid if desired.